Properly link flecs library

engine/libs/flecs/examples/c/rules/facts/src/main.c

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+#include <facts.h>
+#include <stdio.h>
+
+// This example shows how to use rules for testing facts. A fact is a query that
+// has no variable elements. Consider a regular ECS query like this:
+//   Position, Velocity
+//
+// When written out in full, this query looks like:
+//   Position($This), Velocity($This)
+//
+// "This" is a (builtin) query variable that is unknown before we evaluate the 
+// query. Therefore this query does not test a fact, we can't know which values
+// This will assume.
+//
+// An example of a fact-checking query is:
+//   IsA(Cat, Animal)
+//
+// This is a fact: the query has no elements that are unknown before evaluating
+// the query. A rule that checks a fact does not return entities, but will
+// instead return the reasons why a fact is true (if it is true).
+
+int main(int argc, char *argv[]) {
+    ecs_world_t *ecs = ecs_init_w_args(argc, argv);
+
+    ECS_TAG(ecs, Likes); // Likes relationship
+
+    ecs_entity_t bob = ecs_new_entity(ecs, "Bob");
+    ecs_entity_t alice = ecs_new_entity(ecs, "Alice");
+    ecs_entity_t john = ecs_new_entity(ecs, "John");
+    ecs_entity_t jane = ecs_new_entity(ecs, "Jane");
+
+    ecs_add_pair(ecs, bob, Likes, alice);
+    ecs_add_pair(ecs, alice, Likes, bob);
+    ecs_add_pair(ecs, john, Likes, jane);
+    ecs_add_pair(ecs, jane, Likes, john);
+    ecs_add_pair(ecs, bob, Likes, jane); // inserting a bit of drama
+
+    // Create a rule that checks if two entities like each other. By itself this
+    // rule is not a fact, but we can use it to check facts by populating both
+    // of its variables.
+    //
+    // The equivalent of this query in the DSL is:
+    //   Likes($X, $Y), Likes($Y, $X)
+    //
+    // Instead of using variables we could have created a rule that referred the
+    // entities directly, but then we would have to create a rule for each
+    // fact, vs reusing a single rule for multiple facts.
+    //
+    // See the setting_variables example for more details
+    ecs_rule_t *friends = ecs_rule(ecs, {
+        .terms = {
+            { 
+                .first.id = Likes, 
+                .src = { .name = "X", .flags = EcsIsVariable },
+                .second = { .name = "Y", .flags = EcsIsVariable }
+            },
+            { 
+                .first.id = Likes, 
+                .src = { .name = "Y", .flags = EcsIsVariable },
+                .second = { .name = "X", .flags = EcsIsVariable }
+            }
+        }
+    });
+
+    // Lookup the index of the variables.
+    int x_var = ecs_rule_find_var(friends, "X");
+    int y_var = ecs_rule_find_var(friends, "Y");
+
+    ecs_iter_t it = ecs_rule_iter(ecs, friends);
+    ecs_iter_set_var(&it, x_var, bob);
+    ecs_iter_set_var(&it, y_var, alice);
+    printf("Are Bob and Alice friends? %s\n",
+        ecs_iter_is_true(&it) ? "Yes" : "No");
+
+    it = ecs_rule_iter(ecs, friends);
+    ecs_iter_set_var(&it, x_var, bob);
+    ecs_iter_set_var(&it, y_var, john);
+    printf("Are Bob and John friends? %s\n",
+        ecs_iter_is_true(&it) ? "Yes" : "No");
+
+    it = ecs_rule_iter(ecs, friends);
+    ecs_iter_set_var(&it, x_var, jane);
+    ecs_iter_set_var(&it, y_var, john);
+    printf("Are Jane and John friends? %s\n",
+        ecs_iter_is_true(&it) ? "Yes" : "No");        
+
+    // It doesn't matter who we assign to X or Y. After the variables are 
+    // substituted, either yields a fact that is true.
+    it = ecs_rule_iter(ecs, friends);
+    ecs_iter_set_var(&it, x_var, john);
+    ecs_iter_set_var(&it, y_var, jane);
+    printf("Are John and Jane friends? %s\n",
+        ecs_iter_is_true(&it) ? "Yes" : "No"); 
+
+    ecs_rule_fini(friends);
+
+    // Output
+    //  Are Bob and Alice friends? Yes
+    //  Are Bob and John friends? No
+    //  Are Jane and John friends? Yes
+    //  Are John and Jane friends? Yes
+
+    return ecs_fini(ecs);
+}